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Plants in arid environments cope with stress from excessive irradiance by physiological photoprotection of the photosynthetic apparatus and by structurally reducing the leaf area exposed to the sun (structural photoprotection). We assessed the ecological relevance of structural photoprotection in two plants of contrasting architecture co-occurring in a semi-arid environment, using the three-dimensional canopy model YPLANT. We compared the role of crown geometry in avoiding excessive radiation, analysed the costs of structural photoprotection in terms of reduction of potential carbon gain, and compared these costs with those due to seasonal constraints of photosynthesis and tissue ageing. The results of the model simulations indicated that canopy architecture of Stipa tenacissima (a tussock grass) and Retama sphaerocarpa (a leafless leguminous shrub) minimized the risk of overheating and photo-oxidative destruction of the photosynthetic apparatus with steeply oriented foliage and moderate self-shading. But this structural photoprotection imposed an increased cost in terms of potential carbon gain. Diurnal and seasonal patterns of light interception by the crown of these plants translated into a simulated potential carbon gain only half that of an equivalent, horizontal photosynthetic surface. This reduction in potential carbon gain, due to irradiance avoidance, was similar to that imposed by water shortage. S. tenacissima, which ceases photosynthetic activity during periods of drought, exhibited more structural avoidance of irradiance than R. sphaerocarpa, which remains active throughout the year. This illustrates the influence of the capacity of plants to utilize light for carbon fixation on the trade-offs between irradiance capture and avoidance. Structural avoidance of excessive radiation efficiently prevents the risk of damage by intense irradiance, has no special maintenance costs, and is biomechanically cheaper than enhanced light harvesting by a horizontal canopy, which points to structural photoprotection as a very effective strategy to cope with high irradiance stress in poor and adverse habitats.

Tradeoffs Between Irradiance Capture and Avoidance in Semi-arid Environments Assessed with a Crown Architecture Model